The present invention relates to cellular communications, and more particularly to the assignment (mapping) of channel numbers to physical frequencies, for use in communications under the global system for mobile communications (GSM).
Various bands are provided by the global system for mobile communications (GSM) standard for cellular communications, including the GSM 900 band (which in turn includes an extension band, called the E-GSM band), R-GSM for railway cellular communications, and, for traditional cellular applications, the DCS 1800 band, the PCS 1900 band, GSM 450, GSM 480, GSM 850 and the newly added GSM 700 band. Each band is divided into two sub-bands, an uplink sub-band and a downlink sub-band. The uplink sub-band is for mobile transmission (to the serving base station), while the downlink sub-band is for base station transmission to a mobile phone. Each sub-band is divided into 200 kHz frequency slots, each such frequency slot being indicated by an ARFCN (Absolute Radio Frequency Channel Number). Each ARFCN is shared by up to eight mobiles, each using it in turn in a time division multiplex mode, i.e. each mobile is assigned a frequency slot and a time slot in a time division multiple access (TDMA) frame. The combination of a succession of time slots (every eighth time slot beginning with a specified time slot, such as every 3rd time slot out of every eight time slots) and a frequency slot (specified by an ARFCN) indicates what is called a physical channel.
The GSM band, which is illustrated in FIG. 1, and the corresponding mapping from channel numbers (i.e. ARFCNs) to physical frequencies is:
Fup(n)=890.2 MHz+0.2*(nxe2x88x921) MHz (1xe2x89xa6nxe2x89xa6124), and
Fdown(n)=935.2 MHz+0.2*(nxe2x88x921) MHz (1xe2x89xa6nxe2x89xa6124).
The extension band is as follows:
Fup(n)=880.2 MHz+0.2*(nxe2x88x921) MHz (1xe2x89xa6nxe2x89xa650), and
Fdown(n)=925.2 MHz+0.2*(nxe2x88x921) MHz (1xe2x89xa6nxe2x89xa650).
In both of these mappings, i.e. in both the GSM band and the extension band, the quantity n is the ARFCN. As mentioned, a physical channel consists of a carrier frequency given by the ARFCN, and every 8th time slot on the frequency, each time slot having a duration of 4.615/8 ms, often denoted as a burst, there being 8 time slots in a TDMA frame, which therefore has duration 4.615 ms.
The total number of ARFCNs currently supported by GSM signaling is 1024. Excluding GSM 700 (i.e. release 4 of the 3GPP standards), a total of 262 ARFCN values are unused. (GSM 900 uses 124+50 ARFCNs; R-GSM uses 20 ARFCNs; DCS 1800 uses 374 (PCS 1900 uses a subset of DCS 1800 numbers); GSM 400 uses 35+35 numbers, and GSM 850 uses 124. The total amounts to 762 ARFCN values.) However, only two separate large blocks of unallocated ARFCNs exist, 341-511 and 886-954, supporting a total bandwidth of 48 MHz (a total of 240 ARFCN values). Allocating 15 MHz or 74 carriers (at 200 kHz per carrier) (and also a 200 kHz guard band for the operating band) for GSM 700 would leave a very limited number of ARFCNs for future frequency bands, not enough to support any new band allocation like the planned 2.5 GHz IMT (International Mobile Telephony) 2000 extension band.
ARFCNs are currently defined with 10 bits. Several signaling messages include ARFCNs. An obvious alternative to extend the ARFCN range would be to use more than 10 bits. However, such a change would also change all messages that include ARFCNs, and would also change many other messages that do not contain ARFCNs but refer to them.
What is needed, therefore, is a new, dynamic ARFCN allocation procedure that would maintain the existing signaling messages, mostly unchanged, but would also make possible supporting significantly wider spectrum allocations compared to the existing fixed ARFCN mapping. Ideally, no changes to information elements referring to 10-bit ARFCNs would be made.
Accordingly, the present invention is an apparatus and corresponding method for providing for a dynamic mapping of channel numbers to physical frequencies for some cells of a cellular telephone system, the method for use in case of a cellular telephone system representing physical frequencies by channel numbers and providing a fixed mapping of channel numbers to physical frequencies, the cellular telephone system composed of a plurality of public land mobile networks each of which provide coverage in a different set of cells of the cellular telephone system, the dynamic mapping to be used by a particular public land mobile network and so for all the cells for which cellular communication is provided by the particular public land mobile network, the method including the steps of: selecting a set of physical frequencies; selecting from the channel numbers a set of channel numbers sufficient in number to correspond, one-to-one, to the selected physical frequencies; determining a dynamic mapping of the selected physical frequencies to the selected set of channel numbers; and providing to mobile phones in the cells of the public land mobile network an information element indicating the dynamic mapping, the information element including information sufficient to determine the first and last channel number of the dynamic mapping and the first and last physical frequency of the dynamic mapping; wherein the step of providing the information element indicating the dynamic mapping is performed in a way that is compatible with an existing channel numbering space, and so allows keeping unchanged any existing signaling messages.
In a further aspect of the invention, the channel numbers are represented using numbers indicated by at most 10 bits.
In another further aspect of the invention, the cellular telephone system has unused channel numbers and unused physical frequencies. In some applications according to this aspect of the invention, in the step of selecting a set of physical frequencies, the physical frequencies are selected from the physical frequencies not used by the public land mobile network but available for use by the public land mobile network. In some of these applications, in step of selecting channel numbers, the channel numbers are selected from the channel numbers not used by the public land mobile network but available for use by the public land mobile network; and in such applications, the dynamic mapping is sometimes specific to the public land mobile network in that the dynamic mapping would provide channel numbers for only the frequencies actually used by the public land mobile network.
In another further aspect of the invention, the information element includes: a first value indicating the first channel number being mapped; a range value indicating the number of channel numbers being mapped in addition to the channel number indicated by the first value; and a first frequency value, indicating the physical frequency to which the first channel number is being mapped.
In yet another further aspect of the invention, so as to provide for a change in a dynamic mapping being used by a public land mobile network, a duplicated mapping is broadcast by the public land mobile network in which two non-overlapping sets of channel numbers are both mapped at least in part to a same frequency block, the broadcasting being continued for a predetermined time period assumed to be of sufficient extent that any mobile phones operating in the cells of the public land mobile network will have decoded the duplicated mapping. In a still further aspect of the invention, instead of broadcasting the duplicated mapping, the public land mobile network conveys the duplicated mapping via a point-to-point transmission to each mobile phone using the public land mobile network.
Thus, with the present invention it is possible to maintain all existing signaling messages unchanged. The changes to signaling are limited to providing two new system information messages, one for broadcast and one for dedicated mode. In addition, one GPRS Packet System Information message is extended with relevant information, and the GSM handover command is extended to include dynamic mapping information.